Allow/respect alternative package managers

[itsMapleLeaf]

As of writing, create-convex installs packages with npm without any option to skip it. This is problematic, because if I use another package manager or runtime, I would still have to wait the long process for the install, then delete the lockfile and node_modules folder, then install the packages myself.

Other CLIs ask for a package manager to use (npm, yarn, pnpm, or bun are the usual options), but at the very least, it should let you skip the install.